Many new thermographers, and some "old" ones too, often are confused by reflective surfaces. This image, taken over a second cup of coffee recently, shows the issues pretty clearly.
The ceramic mug is a fairly efficient emitter so the surface temperature of 130.3F can be measured quite reliably. Some of the energy from the warm coffee mug is also being reflected by the ceramic creamer pitcher, where it shows an apparent temperature of 77.1F.
Much more reflective is the silver spoon, recently polished for the sake of the demonstration, which clearly reflects the warm coffee mug immediate behind it. Not so obvious is the other reflection of the mug, in the curved bowl of the spoon, showing an apparent temperature of 90.5F. Like the convex and concave mirrors in a "house of mirrors," curved surfaces can be particularly challenging to understand. But even less obvious, until one thinks about it, is probably the reflection of the cold creamer and, in the middle of the spoon, a bit of the ambient surroundings. "We can reflect 'cold' things and 'ambient' things?" YES!
Of course, reflections are only part of the story! Reflective surfaces are also typically low-emissivity surfaces. If the reflections are "lies" about the temperature of a surface, low-emission is a mere whispering of the truth about surface temperature. Taken together, as they must always be, confusion is often the result. When we correct of emissivity and reflected background temperature, we are trying to make sense of the confusion. For very highly reflective surfaces, like the spoon, that can be extremely challenging if not impossible to do, at least in the field.
New thermographers often have trouble believing a thermal reflection can be seen instantaneously. "Isn't it really heating up the spoon?" they ask. Long ago, as children, these same doubters learned similar lessons about another wavelength of electromagnetic energy that behave in much the same way when they learned about mirrors and the reflection of visual light (and "dark"!).
I hope you enjoy Thinking Thermally over your second cup this morning.
Thermally Yours,
John
ASNT NDT Level III #48166
The Snell Group
www.thesnellgroup.com
www.thermalsolutions.org
800-636-9820